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Paper IV - Coordinate Distortions

This paper proposes that an additional 4 character code be appended to the end of the CTYPEi keyword to specify the nature of any distortion away from the basic algorithm described by the first 8 characters of the CTYPEi value. Currently AST ignores all such codes when reading a FrameSet from a FitsChan (except for the ``-SIP'' code defined by the Spitzer Space Telescope project - see below). This means that a FrameSet can still be read from such headers, but the Mapping which gives the WCS position associated with a given pixel position will reflect only the basic algorithm and will not include the effects of the distortion.

If such a FrameSet is then written out to a FitsChan, the resulting CTYPEi keywords will include no distortion code.



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AST A Library for Handling World Coordinate Systems in Astronomy
Starlink User Note 210
R.F. Warren-Smith & D.S. Berry
24th May 2011
E-mail:ussc@star.rl.ac.uk

Copyright (C) 2009 Science \& Technology Facilities Council